1. Field of Invention
The present invention relates to incandescent lamps having improved luminous efficacy and/or durability due to preventing infrared light from escaping the incandescent filament while permitting visible light to escape.
2. Description of Prior Art
Means for improving the luminous efficacy of incandescent lamps by preventing a substantial part of the infrared energy from escaping the incandescent filament, while permitting visible light to escape substantially without hindrance, have been described in numerous prior art references, such as in the following U.S. Pat. Nos. 1,425,967 to Hoffman; 1,342,894 to Bugbee; 2,859,369 to Williams et al; 4,039,878 to Eijkelenboom et al; 4,160,929 to Thorington et al; 4,283,653 to Brett; 4,366,407 to Walsh; and 4,375,605 to Fontana et al.
All these prior art means are based on using curved surfaces for selective transmission and/or reflection of the radiation coming from the incandescent filament, thereby transmitting substantially all of the visible light while reflecting back onto the filament much of the infrared light.
However, it is difficult to make the curvature of these curved surfaces, and to position the filament in relationship thereto, accurately enough to cause the reflected infrared light to hit the filament. Probably as a result of these difficulties, incandescent lamps uing dichroic reflection means for improving luminous efficacy have not found extensive application.